Abstract
In gliding flight, birds morph their wings and tails to control their flight trajectory and speed. Using high-resolution videogrammetry, we reconstructed accurate and detailed three-dimensional geometries of gliding flights for three raptors (barn owl, Tyto alba; tawny owl, Strix aluco, and goshawk, Accipiter gentilis). Wing shapes were highly repeatable and shoulder actuation was a key component of reconfiguring the overall planform and controlling angle of attack. The three birds shared common spanwise patterns of wing twist, an inverse relationship between twist and peak camber, and held their wings depressed below their shoulder in an anhedral configuration. With increased speed, all three birds tended to reduce camber throughout the wing, and their wings bent in a saddle-shape pattern. A number of morphing features suggest that the coordinated movements of the wing and tail support efficient flight, and that the tail may act to modulate wing camber through indirect aeroelastic control.
Highlights
Birds fly over a large speed range morphing their wings and tail to modulate aerodynamic force production
We defined the axis aligned with the wing chord based on the average flow orientation relative to the wings—and not based on wing morphology—this resulted in the average wing sweep equalling zero
We describe the wing and tail morphing over the flight speed range of 6.6–8.1, 5.2–6.9 and 6.3–8.6 m s−1, in T. alba, S. aluco and A. gentilis, respectively
Summary
Birds fly over a large speed range morphing their wings and tail to modulate aerodynamic force production. We seek to understand how birds morph their wing and tail configuration over a range of self-selected gliding speeds, while maintaining approximate weight support and moment balance. We use this conceptual framework for morphing to assess better the contributions of aeroelastic deflection of the feathers, planform shape change by the forelimb muscles, and articulation between the body and wings or tail; such parameters are critical for minimizing drag, changing the forces and moments produced by wings, and modifying stability [8,17,18,19]. A more detailed description of average wing shape and posture can be found in the electronic supplementary material
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